Some types of phased array antennas require that a large numbers of antenna elements be activated simultaneously. This, of course, demands significant power, which is provided by the main system power supplies. The incremental contribution that each antenna element makes to the composite beam is, of course, a feature of the antenna design and so it is possible to determine in advance which antenna elements to energize and at what voltage magnitude in order to achieve a desired beam steering and tracking. Each antenna element is energized according to the pulse width to be transmitted so that there is an instantaneous demand for the time period when the antenna element is active followed by an inactive period when the antenna elements are waiting for the next transmitting pulse. However, the sudden current surges thus consumed when the antenna element becomes active place a severe demand on the system power supplies. It is therefore clearly desirable to energize the antenna elements in such a manner that the current surges are reduced.
It will be apparent from the foregoing discussion that each antenna element operates as a pulsed or switched load which requires a large supply of power intermittently. Conventional solutions using switching mode power supplies for such a load struggle to avoid the output voltage dropping during the transmission pulse and reflecting the load power requirements to the main system power supplies. The ripple current in the current supplied by the main power supply can cause high radio frequency interference RFT which is reflected on to the main supply source if it is not suppressed.
The circuit that converts source input voltage DC to pulsed AC is known as a switching converter or simply ‘converter’ of which there are two principal types, ‘Buck’ and ‘Boost’ although there are several hybrids and variations. The Buck converter normally converts the voltage down so that the output voltage of the converter is lower than the input voltage to the converter by a factor δ that is equal to the duty cycle of the switch. Duty cycle is the ratio between the duration during each cycle that the switch is ON to the total time between successive pulses, i.e. the period, i.e.
            V      OUT        =          δ      ·              V        IN                  δ    =                            T          ON                T            =                        T          ON                          (                                    T              ON                        +                          T              OFF                                )                    where:                VIN=input voltage;        VOUT=input voltage;        δ=Duty cycle        TON=Time when switch is ON        TOFF=Time when switch is OFF        T=Pulse period=(TON+TOFF)        
The Boost converter converts the voltage up so that the output voltage of the converter is higher than the input voltage by a factor
      1          1      -      δ        ,where δ is equal to the duty cycle of the switch. Since δ is less than 1, this factor is greater than 1.
It thus emerges from the foregoing discussion that regardless of the type of converter that is employed, the output voltage of the converter is a function of the duty cycle of the switch. This allows accurate regulation of the voltage simply by controlling the duty cycle of the switch voltage, and this is easily achieved using pulse width modulation, PWM to control the pulse width during which the switching voltage pulse is ON. Since the period of the switching voltage pulse remains constant, adjusting the pulse width of the ON time varies the duty cycle of the switching voltage.
US 2004/178950 discloses a method of controlling a switching element in a switching regulator power supply of a radar. The method of controlling the switching element comprises only switching the switching element during predetermined time intervals, the predetermined time intervals advantageously being sample intervals of a pulse repetition interval of the radar. Thereby by having knowledge of the time intervals the switching element is switching, being able to remove or diminish any influence the switching can have on the quality of received signals and subsequent processing of these signals.
US 2004/062058 discloses a power conversion unit and method for efficient conversion of power for one or more variable loads such as a radar system. Power having a first form is supplied to one or more power conversion units (PCUs) connected to the one or more variable loads. The PCUs are adapted to convert the power from the first form to other forms suitable for use by the components of the destination system. Based at least in part on a predicted load requirement of the variable load, the operation of the PCUs can be controlled to provide sufficient power to the one or more loads at the appropriate time while minimizing wasted power generation by deactivating any unnecessary PCUs during a decrease in power consumption or by activating PCUs during an increase in power consumption. Additionally, based at least in part on a predicted temporary change in the load requirements, the PCU can change its output voltage in anticipation of the temporary change in the load requirement, such as by increasing the output voltage to provide additional energy to the one or more variable loads during a temporary increase in power consumption or by decreasing the output voltage during a temporary decrease in power consumption.
U.S. Pat. No. 5,418,708 discloses a constant power load bank for simulating avionics loads such as pulsing radars on a 270 VDC power system. The load bank is designed to realistically simulate an active aperture radar with 0-100% of the load pulsing while the remainder of the load is either on or off. The pulse controls are designed to simulate any type of pulsing scenario from simple (one control signal) to complex (multiple control signals simulating incremental load application and removal such as an active aperture radar load).
IL 181843 entitled “Controlled power supply and method for pulse load” by the same inventors of the present application and filed Mar. 11, 2007 in the name of the present applicant discloses a method and a controlled power supply for supplying bursts of substantially constant voltage to a switched load via a voltage reservoir, typically constituted by a storage capacitor. Based on a predetermined current that is to be sourced by the load during an active portion of a switching cycle, an average current is computed that should be fed to the voltage reservoir during an inactive portion of the switching cycle to ensure that sufficient energy will stored in the storage capacitor to supply the load without completely draining the storage capacitor. Continuous energy is fed to the storage capacitor at a substantially constant current equal in magnitude to the computed average current.
The complete contents of all the above references are hereby incorporated herein by reference to the extent that they provide useful background. However, since the present invention is a specific application of the power supply described in IL 181843, which has not yet been published, the relevant details of IL 181843 will be described substantially verbatim so as to provide a completely enabling description.
In the related art, an RFI filter at the input of the power supply is used to filter the radio frequency interference so that RFI is not reflected on to the main supply source. Maintaining the ripple current as low as possible also diminishes the conduction losses related to high root mean square (RMS) current values, which reduce the current delivery capability of the supply source. However, when a switch power supply is used in conventional circuits for supplying power as intermittent current bursts, the sudden current burst reflects on the line causing sudden and intermittent voltage reductions on the line. When very high power bursts are being supplied, the RFI filter becomes bulky and expensive.
FIG. 1 shows the topology of a conventional prior art power supply array 10 for feeding DC power to antenna elements 11 of a phased array antenna and FIG. 2 is a table showing typical parameters associated with the power supply array 10. In order to provide a radar system that can track in four directions, four antenna arrays are provided each on a respective “wall” 12, there being one wall 12 for each surface of the system as explained above. Each wall 12 comprises an array of high voltage power supplies 13 that are energized by the system power supplies and each of which feeds high voltage rectified DC voltage to a plurality of switch power supplies 14 via smoothing capacitors 15 coupled at the output of the high voltage power supplies 13 and which serve to reduce voltage ripple of the high voltage power supplies 13. Capacitors 16 at the input to each of the switch power supplies 14, which may be located remote from the high voltage power supplies 13, serve to decouple the switch power supplies 14 from the high voltage power supplies 13. Each of the switch power supplies 14 has a respective output capacitor 17 that feeds voltage to the respective antenna element 11 that serves as a pulse load.
FIG. 2 is a table showing a breakdown of the operating parameters of the power supply array 10 shown in FIG. 1. Thus, starting from the bottom of the table each wall 12 accommodates a single phase array antenna, thus resulting in a total of four phase array antennas. Each of the four walls 12 houses six high voltage power supplies 13, thus resulting in a total of 24 high voltage power supplies 13. Each of the 24 high voltage power supplies 13 is coupled to 27 switch power supplies 14, thus resulting in a total of 648 switch power supplies 14. Each of the switch power supplies 14 supplies 16 antenna elements 11, thus resulting in a total of 10368 antenna elements 11. Now working down from the top of the table, it is assumed that each of the 10368 antenna elements 11 requires that the input voltage across the output capacitor 17 of the corresponding switch power supply 14 is 8.7 volts and it is also assumed that input current (Iinp) to each antenna element 11 is 9 ampere, thus requiring an input power (Pin_p) of 78.3 watts to each antenna element 11, when active. Assuming a 10% duty cycle, this means that when the antenna element 11 is active i.e. draws power from the switch power supply 14, the average power (Pin_avg) drawn by each antenna element 11 is 7.8 watts. The output power (Pout_p) of each antenna element 11 is assumed to be 20 watts based on the efficiency typically achieved by the antenna elements making an efficiency of 26% since the input power (Pin_p) is 78.3 watts.
Having thus determined the operating parameters of each antenna element 11 within each switch power supply 14, we can now work our way down the table and compute the operating parameters of the switch power supplies 14. In like manner, we can then determine the operating parameters of each high voltage power supply 13, then of each wall 12 and finally of the complete power supply array 10. Although the results are tabulated in FIG. 2, for the sake of completeness we will now show how the salient results are derived assuming that the input voltage (Vin) to the antenna is 270 volts.
The output power (Pout_p) of each switch power supply 14 is equal to the power (78.3 watts) fed to each antenna element 11 multiplied by the number (16) of antenna elements 11 in each switch power supply 14, i.e. 1252.8 watts. The input power (Pin_p) to each switch power supply 14 is equal to the output power (Pout_p) divided by the efficiency, estimated at 85% this being a typical efficiency of a switching mode power supply, i.e. 1,474 watts. The input current (Iinp) to each switch power supply 14 is equal to the input power (Pin_p) i.e. 1,474 watts divided by the input voltage (Vin) assumed to be 70 volts, this value being selected to keep the capacitor voltage low enough and avoid large currents, thus making the input current (Iinp) equal to 21.1 ampère.
Similarly, the output power (Pout_p) of each high voltage power supply 13 is equal to the power (1,474 watts) fed to each switch power supply 14 multiplied by the number (27) of switch power supplies 14 in each high voltage power supply 13, i.e. 39,795 watts. The input power (Pin_p) to each high voltage power supply 13 is equal to the output power (Pout_p) divided by the efficiency, again estimated at 85%, i.e. 46,817 watts. The input current (Iinp) to each high voltage power supply 13 is equal to the input power (Pin_p) i.e. 46,817 watts divided by the input voltage (Vin) assumed to be 270 volts this being approximately equal to the voltage obtained by a 3-phase full wave rectifier of a 115V system (i.e. 115*√{square root over (2)}*√{square root over (3)}), thus making the input current (Iinp) equal to 173.4 ampère.
By similar reasoning it can be shown that the output power (Pout_p) of each wall 12 is equal to the power (46,817 watts) fed to each high voltage power supply 13 multiplied by the number (6) of high voltage power supplies 13 in each wall 12, i.e. 280,905 watts. The input power (Pin_p) to each wall 12 is equal to the output power (P_p) divided by the efficiency, estimated at 99% owing to wires and connector losses, i.e. 283,742 watts. The input current (Iinp) to each wall 12 is equal to the input power (Pin_p) i.e. 283,742 watts divided by the input voltage (Vin), again assumed to be 270 volts, thus making the input current (Iinp) to each wall 12 equal to 1,051 ampère.
Finally, since the complete phase array antenna comprises four walls, it can be shown that the output power (Pout_p) of the complete antenna is equal to the power (283,742 watts) fed to each wall 12 multiplied by the number (4) of walls 12 in the complete antenna, i.e. 1,134,968 watts. The input power (Pin_p) to the complete antenna is equal to the output power (Pout_p) divided by the efficiency, assumed to be 100%, i.e. 1,134,968 watts. The input current (Iinp) to the complete antenna is equal to the input power (Pin_p) i.e. 1,134,968 watts divided by the input voltage (Vin), assumed to be 270 volts, thus making the input current (Iinp) to the complete antenna equal to 4,204 ampère.
Having established the operating parameters of the power supply array 10 and its sub-components, we can now calculate the values of the capacitors 15, 16 and 17 as follows.
The energy stored in a capacitor C charged to a voltage V is given by:E=0.5*C*V2  (1)
The energy required by a power pulse of amplitude W and duration (width) tw is given by:E=W*tw  (2)
Given that the energy is delivered to the load by discharging the energy stored in a capacitor from an initial voltage Vi to final voltage Vf, the amount of energy thus required is obtained by:E=0.5*C*(Vi2−Vf2)  (3)
Assuming that for a pulse transmitter, the allowable time to restore the delivered energy to the storage capacitor is a single pulse repetition interval (PRI), this can be achieved by feeding current from a current source into a storage capacitor, so that the integrated current during a single PRI fully charges the capacitor. In this case, the value of the required capacitor is given by:
                    C        =                  E                      0.5            *                          (                                                V                  i                  2                                -                                  V                  f                  2                                            )                                                          (        4        )            
This equation assumes that the efficiency is 100%. But in practice the efficiency is less than 100% and therefore equation (4) must be modified as follows:
                    C        =                              E                          0.5              *                              (                                                      V                    i                    2                                    -                                      V                    f                    2                                                  )                                              *                      1            η                                              (        5        )            
where η is the efficiency. In saying this, it is to be noted that in the following analysis the efficiency, η, does not refer to the efficiency of the capacitor, which is assumed to be 100%, but rather to the efficiency of power conversion between the high voltage power supplies 13 and the switch power supply 14 to which the output capacitor is connected. This distinction is important because when the initial voltage Vi used in equation (4) is directly derived from the voltage of the switch power supply 14, the efficiency, η, may be assumed to be 100%. On the other hand, when the initial voltage Vi used in equation (4) is derived from the voltage of the high voltage power supplies 13, the conversion efficiency, η, which of course is less than 100%, must be factored in.
By substituting for E from equation (2) into equation (4) we obtain:
                    C        =                                            W              *                              t                w                                                    0.5              *                              (                                                      V                    i                    2                                    -                                      V                    f                    2                                                  )                                              *                      1            η                                              (        6        )            
For example if the required output pulse power is 5 KW and the transmitted pulse is 100 μsec width, and the allowed voltage drop across an input capacitor charged to an initial voltage of 70V is 50V (i.e. Vf=20V), then using equation (6) and assuming an efficiency η of 100%, it can be shown that the value of the required storage capacitor is 220 μF.
We have already determined that in the power supply array 10 shown in FIG. 1, the input power (Pin_p) for each high voltage power supply 13 is 46,817 watts. So, by same reasoning, if the width of the transmitted pulse is 100 μsec and the permitted voltage drop across the storage capacitor having an initial voltage of 270V is 2V, then using equation (6) and assuming an efficiency η of 85%, it can be shown that the value of the required storage capacitor is given by.
                    C        =                                                            39                ⁢                                  ,                                ⁢                795                *                100                                            0.5                *                                  (                                                            270                      2                                        -                                          268                      2                                                        )                                                      ⁢            µF                    =                      7            ⁢                          ,                        ⁢            397            ⁢                                                  ⁢            µF                                              (        7        )            
It should be understood that while the efficiency, η of 85% does not appear discretely in equation (7), it is taken into account by virtue of the fact that the input power (Pin_p) for the high voltage power supply 13 is 46,817 watts, while the output power (Pout_p) is 39,795 watts, which is equivalent to an efficiency of 85%.
In other words, each storage capacitor 15 in the power supply array 10 must be rated over 7,000 μF at 300V. Each such capacitor is huge and bulky and there are some 24 such capacitors required in total, i.e. one for each high voltage power supply 13.
Likewise, assuming that the output capacitor 17 for each antenna element 11 operating at 8.7V and an RF output peak power of 20 W and assuming an efficiency η of 26%, may be subjected to a voltage drop of 0.5V and a 15 μs recovery time the value of the output capacitor 17 is given by:
                    C        =                                                            20                *                15                                            0.5                *                                  (                                                            8.7                      2                                        -                                          8.2                      2                                                        )                                                      *                          1              0.26                        ⁢            µF                    =                      277            ⁢                                                  ⁢            µF                                              (        8        )            
The efficiency, η of 26% must be taken into account in equation (8) because the power of 20 W is the output power of the capacitor that is fed to the antenna element, while the initial voltage of 8.7V is derived from the switch power supply 14. Therefore, the efficiency in converting the input power (Pin_p) of the switch power supply 14 (i.e. 78 W) to the output power (20 W) fed to the antenna element must be factored in.
In the power supply array 10 shown in FIG. 1 where there are 10368 antenna elements 11 in total, some 10368 such output capacitors are required.
Yet a further drawback with such a circuit topology where 16 antenna elements are powered by each switch power supply 14 is that failure of a switch power supply 14 results in 16 antenna elements becoming inoperative and this, of course, may impact adversely on the magnitude and shape of the antenna beam. This drawback may to some extent be mitigated by powering antenna elements that can never be energized simultaneously owing to their being on mutually opposing walls among different switch power supplies. However, while this reduces the adverse effect of such a failure it still results in multiple antenna elements becoming inoperative in the event of a failure in a switch power supply 14.
It would therefore be desirable to provide a power supply array for energizing antenna elements of a phase array antenna wherein much smaller input and output capacitors may be used and which lends itself more efficiently to independent operation of each antenna element so as to reduce the number of inoperative multiple antenna elements in the event of a faulty switch power supply.